Mercury compounds, especially alkyl mercurials, are extremely toxic to living organisms. The ability of bacteria to transform mercury compounds from the organic to the inorganic form and vice versa has been examined in freshwater sediment systems, saltwater sediment systems, and in the laboratory. We propose to study, in experimental animals, whether the presence in the intestine of bacteria that can reduce mercury compounds alters their toxicity, distribution in the body, or rate of elimination. The effect of ingestion of mercury compounds on the survival of mercury resistant bacteria in the mouse gut will also be investigated.